With rapid development of electronic industry, the electronic products is further developing towards miniaturization, light-weighting, and high density packing, this greatly promotes the development of a flexible circuit board, in order to achieve integration of elements with wire coupling. The flexible circuit board may be widely used in mobile phone, liquid crystal display, and the field of communication, aerospace, and other industries.
Under promotion by the international market, a functional flexible circuit board is a major part of the flexible circuit board market, and one important index of the functional flexible circuit board is electromagnetic shielding (EMI Shielding). With the functional integration of mobile phone and other communication devices, the components rapidly become to be of higher frequency and higher speed. For example, for the function of the mobile phone, in addition to the original audio transmission function, a photographying function has become an essential function, and WLAN, GPS and internet function have been popularized, along with the integration of sensing elements in the future, the rapid tendency of high-frequency and high-speed of the components is more inevitable. An electromagnetic interference inside or outside the components caused by high-frequency and high-speed as well as signal attenuation, namely insertion loss, during transmission and jitter will gradually become serious.
At present, the shielding film for the circuit board mainly has the following types of structure:
(1) a metal layer is formed on the insulating layer surface, and a conductive adhesive layer is formed on a surface of the metal layer;
(2) a conductive adhesive layer is formed on the surface of a metal layer; There is no essential difference between the structure (1) and (2), and in structure (2) merely no insulating layer is formed on the surface of the metal layer.
(3) an all-direction conductive adhesive layer is formed on the surface of the insulating layer.
The above types of structure may be subdivided to many different structures. For instance, an isolation layer is formed between the metal layers on the basis of the thickness of the metal layers, the number of the metal layers, the pattern of the metal layer (grid or chessboard), forming method of the metal layers and special demand. The shielding film has many structures on the basis of type of the conductive adhesive, type of the conductive particles. However, all the above-described structures have a conductive adhesive layer, and the conductive adhesive layer increases an insertion loss of the circuit board. Meanwhile the conductive metal particles decrease the bending property of the circuit board. Although structure (3) has no metal layer, but the shielding effectiveness is bad, and the insertion loss is great.
The metal layer of the shielding film of the present invention is extremely thin, its thickness is 0.1 to 6 microns, and the bending property of the circuit board can be improved. Meanwhile high shielding effectiveness and low insertion loss can be achieved. Because the conductive particles in the conductive adhesive may increase the insertion loss, the adhesive film layer of the present invention contains no conductive particle, and the grounding is achieved by pressing a novel electromagnetic wave shielding film tightly onto the circuit board in the following manner respectively:
(1) piercing the adhesive film layer by a rough surface of the electromagnetic shielding layer of the shielding film, and connecting with a ground layer of the circuit board, in order to achieve grounding.
(2) piercing the whole electromagnetic shielding film by an electrically conductive substance, and connecting with the ground layer of the circuit board, in order to achieve grounding.
(3) by a mechanical or laser drilling method for circuit board, forming through holes or blind holes on the circuit board comprising the shielding film; and connecting the metal shielding layer with the ground layer of the circuit board by metallizing the holes, to achieve grounding.